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Abstract

Twenty female young adult rats were injected with tritiated thymidine (3HTdR). The animals were then killed in groups of five, at the following times: 1 hour, and 4, 16, and 23 days. Autoradiograms of sections through the submandibular gland were prepared, and the location of labelled cells in relationship to tubuli and acini was recorded. The different tubular and acinar cross sections could be distinguished by their cell number. Narrow tubuli had fewer nuclei than the wider ones. The nuclear number of a cross section was defined as its class and the location of a labelled epithelial cell was expressed in relationship to the class where it was found. The location of a labelled stromal cell was determined by the class of its neighboring tubular or acinar cross sections. The mean cell numbers of intercalated, granular, and striated duct cross sections were, respectively, 4.7, 10.5, and 10.2, while the average cell content of acini was 4.7 cells. One hour after labelling most labelled tubular epithelial and stromal cells were found in tubular cross sections (or low tubular classes), while in the acini, labelled epithelial and stromal cells were found mainly in wider cross sections (or higher acinar classes). Within the next 23 days labelled tubular cells and stroma proceeded into higher classes, while labelled acinar epithelium and stroma cells were displaced into narrower cross sections (or lower classes). The displaced tubular epithelium and stroma covered daily 0.26 classes. At this velocity the cell will reach the highest tubular class in 62 days and the estimated maximal tubular cell life span is 62 days. Acinar epithelium and stroma were displaced at a velocity of 0.024 classes/day, and their maximal life span was 125 days. These results indicate that contrary to the accepted belief, submandibular gland epithelium and stroma are renewing cell populations. Tubular cell progenitors are formed in the intercalated duct, from which they proceed into the granular. During this journey they are accompanied by dedicated stroma cells which are displaced at the same velocity as the epithelial cell. Moreover, the data suggest that the intercalated duct supplies also acinar cell progenitors which are displaced into the acinus. Since in the submandibular gland epithelium and stroma proceed along the tubuli and acini concurrently they probably carry their microenvironment with them.